The general configuration of a dynamic microphone is explained with reference to FIG. 4. The dynamic microphone, which is mainly used for vocals and speech, includes a cylindrical microphone case 10 serving as a grip part. Usually, the microphone case 10 is made of a metal such as brass alloy.
In this example, in the microphone case 10, an internal cylinder 12 is held coaxially via a shock mount member 11 consisting of a rubber elastic body, and a microphone unit 20 is supported on one end side of the internal cylinder 12.
The microphone unit 20 includes a diaphragm 21 having a voice coil 21a, and a magnetic circuit 22. The diaphragm 21 has a center dome and a sub dome formed around the center dome. To the boundary part between the center dome and the sub dome, the voice coil 21a is attached with an adhesive.
The magnetic circuit 22 includes a disc-shaped magnet 22a magnetized in the thickness direction, a bottomed cylindrical yoke 22b arranged on one pole side of the magnet 22a, and a center pole piece 22c arranged on the other pole side of the magnet 22a, and a magnetic gap is formed between the yoke 22b and the center pole piece 22c. 
The diaphragm 21 and the magnetic circuit 22 are assembled to one end side of a unit holder 23 in the state in which the voice coil 21a is arranged oscillatably in the magnetic gap. On the diaphragm 21, a resonator 24 having a front acoustic terminal 24a is put.
In the case of a unidirectional dynamic microphone, the unit holder 23 is provided with a rear acoustic terminal 23a communicating with the back surface of the diaphragm 21. Both of the front acoustic terminal 24a and the rear acoustic terminal 23a are holes for allowing sound waves to pass through.
To the other end side of the unit holder 23, a cap 25 having a sound hole 25a covered with an acoustic resistance material 26 is fitted. Thereby, in the internal cylinder 12, a back air chamber 12a communicating with the back surface side of the diaphragm 21 via the acoustic resistance material 26 is formed.
To one end side of the microphone case 10, a protective cover 27 consisting of a metallic mesh is attached to protect the microphone unit 20 from a drop shock or the like, and on the other end side of the microphone case 10, an output connector 30 is mounted.
The unidirectional dynamic microphone has both of a mass control property and a resistance control property. In contrast, a non-directional dynamic microphone has a resistance control property only. The driving force of a non-directional component is obtained by a difference in pressure between the front surface side (sound receiving surface side) of the diaphragm 21 and the back air chamber 12a existing on the back surface side thereof, and also is controlled by the acoustic resistance of the acoustic resistance material 26.
FIG. 5 shows an acoustic equivalent circuit of the unidirectional dynamic microphone. In FIG. 5, symbol P1 denotes a front sound source, P2 denotes a rear sound source, m0 and s0 denote the mass and stiffness of the diaphragm 21, respectively, r1 denotes the acoustic resistance of the acoustic resistance material 26, s1 denotes the stiffness of the back air chamber 12a, and m1 denotes the mass of the back air chamber 12a. 
In the dynamic microphone, the size (volume) of the back air chamber 12a greatly affects the frequency band of sound capture. For example, if the back air chamber 12a is small, the impedance of the back air chamber 12a increases at a low frequency.
Therefore, considering the capture of sounds to a low range, the dynamic microphone, both unidirectional and non-directional, is designed so that the impedance of the back air chamber 12a is decreased by increasing the volume of the back air chamber 12a. 
However, the increase in volume of the back air chamber 12a hinders the design of other parts. In particular, in a microphone in which an electronic circuit is mounted in the microphone case 10 serving as a grip part, such as a wireless microphone, the volume of the back air chamber 12a cannot be secured sufficiently.
Japanese Patent Application Publication No. S62-000197 describes a dynamic microphone in which the volume of the back air chamber can be changed according to the microphone type. In this dynamic microphone, however, the volume of the back air chamber is changed by a configuration in which a movable partition plate is provided in the back air chamber having a predetermined fixed volume, and the interior of the back air chamber is divided into two by the movable partition plate. Therefore, the volume of the back air chamber itself cannot be increased further.
Accordingly, an object of the present invention is to provide a dynamic microphone in which even in the case where the volume of a back air chamber is small, sounds with a low frequency (low range) can be captured by equivalently decreasing the acoustic impedance of the back air chamber.